class StartButton extends ex.ScreenElement {
  constructor(x: number, y: number) {
    super({
      x,
      y
    });
  }
  onInitialize() {
    this.graphics.use(
      new ex.Rectangle({
        color: ex.Color.Green,
        width: 300,
        height: 50
      })
    );
    // ...
    this.on('pointerup', () => {
      alert("I've been clicked");
    });
    // ...
  }
}

var engine = new ex.Engine({
  width: 800,
  height: 400
});

engine.add(new StartButton(400, 200));

engine.start();
